1. Field of the Invention
The invention relates to a process for preparing linear organic oligomers, semiconductive layers comprising these oligomers and their use in semiconductor technology.
2. Brief Description of the Prior Art
The field of molecular electronics has developed rapidly in the last 15 years with the discovery of organic conductive and semiconductive compounds. In this time, many compounds which have semiconductive or electrooptical properties have been found. While it is generally accepted that molecular electronics will not displace conventional semiconductor building blocks based on silicon, it is believed that molecular electronic components will open up new applications in which suitability for coating large areas, structural flexibility, processability at low temperatures and low costs are required. Semiconductive organic compounds are at present being developed for applications such as organic field effect transistors (OFETs), organic luminescence diodes (OLEDs), sensors and photovoltaic elements. Simple structuring and integration of OFETs into integrated organic semiconductor circuits make it possible to achieve inexpensive solutions for smart cards or price signs which have hitherto not been able to be realized by means of silicon technology because of the price and the lack of flexibility of the silicon building blocks. Likewise, OFETs could be used as switching elements in large-area flexible matrix displays.
In organic field effect transistors, two large classes of compounds have hitherto been used. All of these compounds have long conjugated units and are divided according to molecular weight and structure into conjugated polymers and conjugated oligomers. Here, oligomers are generally distinguished from polymers on the basis of oligomers having a uniform molecular structure and a molecular weight of less than 10000 dalton, and polymers generally having a molecular weight distribution. However, there is a continuous transition between oligomers and polymers. The distinction between oligomers and polymers also often reflects the difference in the processing of these compounds. Oligomers are frequently vaporizable and can be applied to substrates by vapour deposition processes. Compounds which are no longer vaporizable and therefore have to be applied by other processes are frequently referred to as polymers, regardless of their molecular structure.
An important prerequisite for preparing high-quality organic semiconductor circuits is compounds of extremely high purity. Related to this prerequisite is the fact that in semiconductors, ordering phenomena play a major role. Hindrance of a uniform alignment of the compounds and formation of pronounced grain boundaries lead to a dramatic deterioration in the semiconductor properties. As such, organic semiconductor circuits which have been built up using compounds which do not have an extremely high purity are generally unusable. Residual impurities can, for example, inject charges into the semiconductive compound (“doping”) and thus decrease the On/Off ratio or can serve as charge scavengers and thus drastically reduce the mobility. Furthermore, impurities can initiate reaction of the semiconductive compounds with oxygen and oxidizing impurities can oxidize the semiconductive compounds and thus decrease possible storage, processing and operating lives.
Vaporizable compounds generally have the advantage that further purification occurs during the vaporization step and impurities which are difficult to vaporize are not applied. However, contamination by small amounts of other compounds which are very similar to the desired compounds and vaporize in a similar way cannot be eliminated in this way. It is therefore, in particular, necessary to provide vaporizable compounds which do not contain such impurities.
Important representatives of oligomeric semiconductive compounds are, for example, oligothiophenes, in particular those having terminal alkyl substituents (Adv. Mater., 2002, Volume 14, p. 99). The use of such oligothiophenes for producing organic field effect transistors is described, for example, in WO-A 92/01313 and JP-A 04 133 351.
There have been a series of attempts to prepare oligothiophenes of sufficient purity. Thus, EP-A 402 269 describes the preparation of oligothiophenes by oxidative coupling, for example using iron chloride (p. 7, lines 20-30, p. 9, lines 45-55).
However, this synthetic method leads to oligothiophenes which are present in the cationic form, also referred to among specialists as the doped form (EP-A 402 269, p. 8, lines 28-29). These oligothiophenes are as a result unusable for applications in semiconductor electronics, since the cationic form of the oligothiophenes conducts electric current well but displays no semiconductor effect. Although it is possible to reduce cationic oligothiophenes, e.g. by means of an electrochemical or chemical reaction, this is complicated and does not lead to the desired result.
An alternative is the coupling of organolithium compounds using iron(III) salts, e.g. iron(III) chloride. This reaction generally gives undoped, i.e. uncharged, oligothiophenes, but secondary reactions accompanying this reaction lead to products which are heavily contaminated with iron and chlorine. Iron(III) compounds other than iron(III) chloride, for example iron(III) acetylacetonate, have been proposed as coupling reagents (J. Am. Chem. Soc., 1993, 115, 12214). However, owing to the lower reactivity of this coupling reagent, this variant has the disadvantage that the reaction has to be carried out at elevated temperature. The higher temperature promotes lithium-hydrogen exchange and the secondary reactions occurring as a result make it impossible to obtain high-quality oligothiophenes even by means of intensive purification operations (Chem. Mater., 1995, 7, 2235).
Syntheses using Grignard compounds (JP-A 02 250 881) or organozinc compounds (U.S. Pat. No. 5,546,889) in the presence of nickel catalysts likewise lead to products which have to be purified at high cost.
A further possible way of preparing oligothiophenes which has been described in the literature is oxidative coupling by means of copper salts, in particular by means of copper(II) chloride. Thus, Kagan et al. describes the oxidative coupling of 2-lithiothiophenes in the presence of copper(II) chloride in dimethylformamide/tetrahydrofuran (Heterocycles, 1983, 20, 1937). Some improvements in the procedure, e.g. the use of complexed lithium alkyl compounds as lithiation reagents, have been proposed. However, it has been found in the preparation of, for example, sexithiophene that the product still contains 0.77% by weight of chlorine and 0.033% by weight of copper after purification by recrystallization. Of these impurities, at least the chlorine is at least partly chemically bound to the oligothiophene and cannot be removed further even by means of further complicated purification (Katz et al., Chem. Mater., 1995, 7, 2235).
There is therefore a continuing need for an improved process for preparing organic oligomers, in particular oligothiophenes, which have only very small amounts of contamination and thus few defects. In particular, there is a need for such processes for preparing organic oligomers which give the oligomers in high quality for use as semiconductors without additional complicated purification operations.
It is therefore an object of the present invention to provide such a process.